There has been a considerable body of literature reported describing attempts to produce composite materials with very high thermal conductivity, e.g., for use in high power electronic packaging and other thermal management applications. Much of this literature relates to the addition of particulate fillers to a metal matrix, forming a metal matrix composite (MMC). The benefit of adding a particulate filler with a high thermal conductivity to a metal to form a metal matrix composite is well known. Properties of the MMC can often be optimized to suit the requirements of a particular application by properly selecting the properties of the particulate filler and metal matrix. Examples would include the addition of SiC particles to an Aluminum matrix. The SiC is readily wet by molten aluminum and aluminum alloys when it comes in contact with the filler particles. The Al/SiC composites have been reported to achieve strengths of 400 MPa at a filler loading of >40 vol. %, indicating a good bond was formed between the SiC particles and Al. MMC's comprised of particulate SiC and an aluminum matrix have advantages over pure Al structures in terms of coefficient of thermal expansion (CTE), stiffness, and wear resistance.
In general, however, the thermal conductivity of the Al/SiC MMC's do not meet desired expectations. Thermal conductivity of pure aluminum is ˜200 W/m.k, and the thermal conductivity of pure crystalline SiC particles is ˜320 W/m.k. Values of thermal conductivity for Al/SiC MMC's are generally <200 W/m.k, and typically <180 W/m.k (ref 1-5). These Al/SiC MMC's were consolidated by processes such as stir casting, powder metallurgy, or low pressure and pressureless melt infiltration. These methods are relatively slow, and have a considerable residence time when the aluminum is in the molten state, allowing the SiC to react with the molten Al forming aluminum carbide. An example reaction is:Al+3SiC═Al4C3+3Si  (1)For that reason, these processes generally require the use of Al—Si alloys which decrease the activity of the Si and reduce the kinetics of the adverse carbide reaction during long contact times with the molten aluminum. These Al—Si alloys generally have a lower thermal conductivity than pure aluminum, thus reducing the thermal conductivity of the SiC/Al MMC. Alternatively, the use of rapid high pressure metal infiltration (also referred to as squeeze casting) to consolidate the particulate reinforced aluminum composites results in a much faster consolidation of the composite. Exposure times of the particles to the molten aluminum are generally seconds as opposed to hours for the non-pressure processes described above. As a result of the rapid consolidation with squeeze casting, pure aluminum can be used, and thermal conductivities of up to about 225 W/m.k would be expected for SiC loadings of ˜55 vol. % in the composite.
Certain properties of diamond make it particularly attractive as a possible filler for high thermal conductivity MMC's. The thermal conductivity of diamond is about 700-2000 W/m.k, depending on crystalline perfection. It also has a low CTE (approximately 1 p.p.m./degree centigrade). However, researchers using consolidation processes for diamond/aluminum MMC's with a long exposure time for diamond contact with the molten aluminum have been unable to obtain high levels of thermal conductivity. Composites comprising an aluminum matrix containing 50 vol. % of industrial diamond particles have been reported to have a thermal conductivity <200 W/m.k (Johnson and Sonuparlac, ref 3). A microstructural examination of the diamonds in the composite revealed the presence of a thick surface layer of aluminum carbide (Al4C3) on the diamond particles. This surface layer is formed by the reaction shown in equation (2).3C+4Al═Al4C3  (2)Aluminum carbide is generally recognized to have low thermal conductivity, and is hydroscopic. The diamond particles with the thick layer of aluminum carbide formed on the surface, in effect, function more as an aluminum carbide particle than a diamond particle, resulting in poor thermal conductivity for the composite.
Coating the diamond particles with a protective layer before contacting the diamond particles with molten aluminum forming the aluminum composite can prevent the reaction to form Al4C3. The application of a distinct SiC coating on diamond particles and subsequent composite formation with Al has been described in the literature (ref 6). A loose bed of industrial diamond powder (Beta Diamond Products), with a particle size of 40-50 microns, was coated with SiC using a chemical vapor deposition process of the diamond particle array, which was termed chemical vapor infiltration, or CVI, by the authors. In this CVI process, a distinct SiC coating is applied or deposited on the surface of the diamond particles. (It is known in the art that the deposition of SiC by the CVI process occurs at about 1000 degrees centigrade.) Johnson and Sonuparlac estimated the thickness of the SiC coating varied between 0.41 to 1.6 microns, depending on process conditions. They further estimated the total SiC content of the coated diamond particles at 3% to 11% by volume. The preform particle arrays were observed to have stiffened by the CVI SiC coating. The SiC coated diamond preforms were infiltrated by a pressureless metal infiltration process, using an Al-15Si-5Mg wt. % alloy. Process conditions were optimized to assure complete infiltration of the preform. The relevant properties of the MMC's are shown in Table I.
TABLE IPhysical properties of ~50 vol. % diamond/AlMMC composites described in reference 6.CoatingAl4C3ThermalYoung'sThicknesscontentDensityConductivityCTEmodulus(Microns)(wt %)(gm/cc)(W/m-K)(ppm/K)(Mpa)0.410.0783.1682396.83680.530.0713.1612426.53850.970.0533.132595.24071.210.0473.1251315.94081.230.0733.222404.63981.420.123.2132255.04131.60.0933.162344.5427
At all levels of SiC coating thickness, it appears the formation of Al4C3 on the diamond particle has been reduced to a very low level, but remains greater than zero. The densities reported are consistent with the author's claim of full infiltration of the SiC coated diamond preforms with the aluminum alloys. The thermal conductivity values of the composite, ranging from 131-259 W/m.k, however, are very low considering the relatively high loading of diamond particles in the composite (40-50 vol. %), and the thermal conductivity of the diamond. There is no apparent relationship between SiC coating thickness on the diamond particles and the thermal conductivity of the composite. The authors of that work indicated they believe that the increased stiffness (Young's modulus) observed with increased SiC coating thickness on the diamond particles is due to the formation of SiC bridges between the diamond particles. These results seem to indicate that the SiC coating thickness may be excessive, thereby causing bridging between the diamond particles. However, with the CVI process, it is difficult to obtain a thin uniform SiC coating on the diamond particles, i.e. covering 100% of the diamond surface.
Unlike the above described prior art, the instant inventors have used rapid high pressure melt infiltration (squeeze casting) to prepare composites of aluminum with uncoated diamonds. As described above, this process reduces exposure time of the diamond to the molten aluminum to a very short time (e.g., <2 seconds). Thermal conductivity of the composite was still <200 W/m.k, even though there was expected to be minimal aluminum carbide at the surface of the diamond particles.